Z-scheme microbial photoelectrochemical system (MPS) for wastewater-to-chemical fuel conversion

ABSTRACT

A wastewater to chemical fuel conversion device is provided that includes a housing having a first chamber and a second chamber, where the first chamber includes a bio-photoanode, where the second chamber includes a photocathode, where a backside of the bio-photoanode abuts a first side of a planatized fluorine doped tin oxide (FTO) glass, where a backside of the photocathode abuts a second side of the FTO glass, where a proton exchange membrane separates the first chamber from the second chamber, where the first chamber includes a wastewater input and a reclaimed water output, where the second chamber includes a solar light input and a H 2  gas output, where the solar light input is disposed for solar light illumination of the first chamber and the second chamber.

FIELD OF THE INVENTION

The current invention relates to microbial electrohydrogenesis. Morespecifically, the invention relates to solar-assisted microbialelectrohydrogenesis by integrating two semiconductor photoelectrodeswith a conventional microbial fuel cell (MFC) device.

BACKGROUND OF THE INVENTION

With the drastic increase of human population, there is an ever-growingdemand for energy and clean water for the continuous economic growth andsuitable inhabitation on earth. Over the years, federal government hasapplied distinct strategies to address these two needs separately: themunicipal wastewater is collected by local wastewater plants forpurification and subsequent reuse as reclaimed water, while the energysource is mainly based on natural gas, and crude oil. Apparently, thesetwo strategies are decoupled. Millions tons of wastewater is producedfrom industrial and agricultural operations each year, and about 25billion US dollars are spent annually for wastewater treatment in theUnited States alone.² Meanwhile, the use of natural gas/petroleumgenerates a lot of greenhouse gas and toxic chemicals, which poses aserious threat to the environment, and also leads to additional cost totreat the pollution.

FIG. 1 shows a schematic drawing integrating photocatalysis withmicrobial metabolism to remediate wastewater and produce chemical fuels.The wastewater treatment and energy recovery can simultaneously beachieved by microbial fuel cell (MFC) technology. For instance,microbial electrohydrogenesis process has been experimentallydemonstrated in microbial electrolysis cell (MEC) using a wide range ofmicroorganisms with various organic nutrients to produced hydrogen.However, thermodynamic constraints limit microbial electrogensiss andhydrogen production occur simultaneously without the addition of anexternal bias. To overcome the thermodynamic constrains, an externalbias is usually applied to sustain the current/hydrogen generation.Nevertheless, the need of external bias reduces the overall energyrecovery ratio and adds to the complexity and cost for hydrogenproduction, making microbial electro-hydrogenesis less attractive as anenergy solution. Considerable efforts have been made on optimization ofMEC reactors, design of anodes, and catalysts to reduce theabove-mentioned energy losses. Alternatively, to obtain the requiredenergy from a renewable energy source is also a promising approach thatcan fundamentally address the issue.

Previously reported is a dye-sensitized solar cell (DSSC)-poweredmicrobial electrolysis cell (MEC). The MEC was a conventional dualchamber device with the anode inoculated with anaerobic digester sludgefrom a sewage treatment plant and acetate was fed as the electron donor.The MEC was integrated with a conventional DSSC device composed of aruthenium dye-loaded TiO₂ nanoparticle film as working electrode and aplatinized FTO glass as counter electrode. The DSSC device harvestssunlight to provide the required energy for hydrogen production.However, Ru is a rare and expensive element, which renders this approachto be unsustainable.

A prior art hybrid device is shown that includes a photoelectrochemicalcell (PEC) device and a MFC device. Significantly, this hybrid devicegenerates hydrogen gas at zero external bias using biodegradable organicmatters and sunlight as the only energy sources. Shown in FIG. 2A, is aprior art PEC device composed of a TiO₂ photoanode and a Pt cathode. TheMFC is an air-cathode dual-chamber device, inoculated with eitherShewanella oneidensis MR-1 (batch-fed on artificial growth medium) ornatural microbial communities (batch-fed on local municipal wastewater).Under light illumination, the TiO₂ photoanode provides a photovoltage of˜0.7 V that overcomes the thermodynamic barrier for microbialelectrohydrogenesis. As a result, a pronounced current generation andsustainable production of hydrogen gas (FIG. 2B). This hybrid device(with wastewater as anolyte) achieved only a decent solar conversionefficiency of ˜1% at zero external bias under one sun illumination, andonly fair soluble chemical oxygen demand (SCOD) removal rate of ˜200mg/L/day, which is comparable to the efficiency of some conventionalmicrobial devices. The originally black wastewater can eventually turninto almost clear solution. Taken together, this hybrid device holdsgreat promise of being set up in remote/rural areas, without electricityand fuel supplies, for self-sustained wastewater treatment and chemicalfuel production.

A solar-assisted microbial device has been successfully demonstrated bythe inventors. For instance, the hybrid MFC-PEC device achieved theoverall solar-to-hydrogen conversion efficiency of ˜1%, which is verypromising given that the device was operated in a sustainable mannerusing sunlight and wastewater as the only energy sources. What is neededis improvement in the performance of an MPS by enhancing the chargegeneration and collection processes.

There is urgent need to employ energy-efficient processes for wastewatertreatment, and simultaneously recover the “wasted energy” contained asorganic matters in wastewater.

SUMMARY OF THE INVENTION

To address the needs in the art, a wastewater to chemical fuelconversion device is provided that includes a housing having a firstchamber and a second chamber, where the first chamber includes abio-photoanode, where the second chamber includes a photocathode, wherea backside of the bio-photoanode abuts a first side of a planatizedfluorine doped tin oxide (FTO) glass, where a backside of thephotocathode abuts a second side of the FTO glass, where a protonexchange membrane separates the first chamber from the second chamber,where the first chamber includes a wastewater input and a reclaimedwater output, where the second chamber includes a solar light input anda H₂ gas output, where the solar light input is disposed for solar lightillumination of the first chamber and the second chamber.

In one aspect of the invention, the bio-photoanode includes hematite(α-Fe₂O₃) nanowires.

In another aspect of the invention, the bio-photoanode includeselectrogenic bacterial strains.

In a further aspect of the invention, the bio-photoanode has asemiconductor material that can include TiO₂, Fe₂O₃, WO₃, ZnO, or BiVO₄.

According to one aspect of the invention, the photocathode has asemiconductor material that can include InGaN, GaN, InP, GaP, Si, Cu₂O,or CuBi₂O₄.

In yet another aspect of the invention, the photocathode is compatiblewith an anoxic buffered solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing integrating photocatalysis withmicrobial metabolism to remediate wastewater and produce chemical fuels.

FIGS. 2A-2B show 2A a schematic configuration of a prior art MPC device,wherein shown are the electrons generated from photoanode and bacteria,respectively, and 2B shows a linear sweep voltammograms collected from aPEC device and a PEC-MFC device, at a scan rate of 20 mV/s in the darkand under one sun illumination (100 mW/cm²).

FIGS. 3A-3B show 3A SEM image of carbon aerogel, and 3B pore sizedistribution.

FIGS. 4A-4B show 3A a schematic configuration of a hematite basedbio-photoanode device highlighting the electrons generated fromphotoanode, bacteria and electron shuttle, with the photoexcited holesgenerated from photoanode, 4B an example of linear sweep voltammogramscollected from a hematite based bio-photoanode device in the presence oflive bacteria, dead bacteria, and absence of bacteria, at a scan rate of20 mV/s in the dark (dashed lines) and under one sun illumination (100mW/cm², solid lines).

FIG. 5 shows a schematic diagram illustrating the mechanism of solarassisted microbial electro-hydrogenesis using a photoanode.

FIGS. 6A-6B show 6A a schematic configuration of a Z-scheme MPS, and 6Bthe corresponding energy diagram illustrates the carrier generation andtransfer in the device, where the solid and empty dots representelectrons and holes, respectively, and further showing the electronsgenerated from bio-photoanode and photocathode, respectively, and thedashed lines represent the equilibrium Fermi level, according to oneembodiment of the invention.

FIG. 7 shows a schematic diagram of the band-gap and relative energiesof different semiconductors in terms of vacuum level and NHE, accordingto one embodiment of the invention.

FIG. 8 shows a schematic diagram of a free-standing Z-scheme MPS,according to one embodiment of the invention.

DETAILED DESCRIPTION

The current invention provides a self-sustained microbial photochemicalsystem (MPS) that removes soluble chemical oxygen demand (SCOD) inwastewater and simultaneously recovers the “wasted energy” stored in theorganic wastes for photochemical generation of chemical fuels. Providedis a hydrogen gas production, solar-assisted microbial device usingmunicipal wastewater and sunlight as the sole energy sources. Accordingto one embodiment, the invention provides fundamentally new MPSarchitecture referred to herein as a “Z-scheme” MPS, in which abio-photoanode is interfaced with a semiconductor photocathode. Theinvention provides chemical fuel generation in a sustainable manner byusing a Z-scheme MPS that operates in an outdoor environment undernatural sunlight illumination with continuous flow of wastewater.

The current invention provides solar-assisted microbialelectrohydrogenesis by integrating semiconductor photoelectrode withconventional MFC device. The current invention couples a photocathodewith bioanode by matching the redox potentials of bacterial cells andthe electronic bands of semiconductor. In one embodiment, the devicegenerates a pronounced current in short-circuit configuration (measuredat zero external bias) under modest white light illumination of 20mW/cm².

To improve device performance, the invention incorporates a bio-anodewith semiconductor material in a MFC device, where the amount of chargesproduced on bio-anode and photoelectrode are matched. Until now, thecharge generation and collection on the bio-anode was the limitingfactor for the overall efficiency of the microbial electrohydrogenesisreaction. The current invention addresses this issue through an increasein the availability of “bio-electrons” by increasing the number and/orthe activity of microorganism. Further, the overall efficiency isimproved by enhancing the charge collection efficiency of the bio-anode.The latter aspect is particularly important for the MPS operated incontinuous flow mode, where the availability of bio-electrons istypically in excess. The current invention increases the collectionefficiency of bio-electrons from a material perspective.

A key aspect of the solar-assisted microbial device of the currentinvention is the utilization of solar energy to facilitate the microbialprocess. Here, the solar light harvesting capability of thephotoelectrode is central to the success of the solar-microbialapproach.

Previously, the light harvesting capability and photovoltage of thesolar-microbial devices was limited by the single band-gap system. Thecurrent invention increases the photovoltage and light absorption of MPSby providing a fundamentally new device of a “free-standing” MPS, whichrequires neither electrodes nor ion exchange membrane. According to oneembodiment hydrogen gas is generated photochemically in a sustainablemanner when the free-standing MPS is dispersed into wastewater undersunlight illumination.

According to one embodiment, a cost-effective microbial device isprovided that increases efficiency, e.g. SCOD removal rate and powerproduction, and lowers the material, fabrication and operation cost. Thecurrent invention improves the energy conversion efficiency of MPS usingfree-standing MPS for photochemical hydrogen production for providing asustainable energy solution.

Previous MFC studies have primarily focused on the modification ofmicroorganisms and culture conditions, in order to increase thegeneration of bio-electrons from biological approach. The currentinvention relies on the fact that the electron transfer between theelectrode and bacteria plays an equally important role in determiningthe device efficiency. For instance, the effective surface area,electrical conductivity and chemical nature of the electrode are alldirectly related to the charge collection efficiency of bio-anode, andthus, the current generation of MFCs.

The collection efficiency of bio-electrons depends on two key factors,the contact area between bacteria and electrode as well as the chargetransfer rate at the interface. The current invention addresses this intwo approaches. First, a three-dimensional (3D) conductive nanomaterialis provided for the bio-anode. In comparison to conventional carbonelectrodes (e.g., carbon cloth, carbon felt, and carbon paper), the 3Delectrode provides not only a larger accessible surface area formicrobial colonization and electron mediators, but also a uniformmacro-porous scaffold for effective mass diffusion of the culturemedium. Second, the bacteria/electrode interfacial charge transfer isincreased by replacing the conventional carbon electrodes with asemiconductor photoanode. Under light illumination, the photoelectrodeprovides a large driving force for bio-electron transfer from bacteriato the electrode.

Turning now to the 3D bio-anode, carbon-based materials such as carboncloth, carbon paper, carbon felt and graphite brush are most commonlyused anode materials for MFCs. These commercially available electrodematerials are chemically inert, highly conductive and inexpensive.However, these microstructures have relatively small surface area formicrobial colonization, and thus, limit the power density of MFC device.The current invention enhances the MFC by modifying the electrodes withnanostructures to increase the accessible surface area for bacterialcolonization. A flexible MFC anode has been investigated by employingnickel foam as a 3D conducting scaffold and coated with reduced grapheneoxide sheets to increase its accessible surface area for bacteria andelectron mediators. The 3D anode produces a substantially enhancedvolumetric power density than that of plain nickel foam and conventionalcarbon based electrodes measured in the same conditions. Nevertheless,the relatively high cost and heavy weight of nickel foam compared tocarbon materials limit its application as electrode. To address this alow-cost carbon aerogel material is provided as an electrode for MPS,according to one embodiment. Carbon aerogel is a 3D conductive scaffoldwith very low mass density (0.18 to 10 mg/cm²), high porosity (over 50%)and extremely large surface areas (up to ˜3000 m²/g). The cost of carbonaerogel is estimated to be less than $1 per dm³. The mixture of a numberof different size of pores in carbon aerogel is beneficial for servingas a bio-electrode. The large pores with diameter >500 nm allow forefficient diffusion of bacteria, while the small pores enhance theinterfacial surface area between electrode and molecular electronshutters present in the solution. The carbon aerogel electrodeoutperforms the conventional carbon based electrodes as well aspreviously reported 3D electrodes. A carbon aerogel with macro-poresranging from 0.5 to 10 μm as the conductive scaffold for bacteriacolonization has been synthesized, as shown in FIGS. 3A-3B. Thislow-cost hydrothermal fabrication method used is also feasible to scaleup the fabrication of carbon aerogel for MPS with increasing scale. Thesize of carbon aerogel is only practically limited by the size ofcontainer for hydrothermal reaction.

In one embodiment, n-type semiconductor materials are provided as abio-photoanode to replace the conventional carbon electrode. Thebio-photoanode not only extracts “bio-electrons” from bacteria, but alsogenerates photoexcited electrons by harvesting sunlight. A key aspect ofthe invention is a microbial device utilizing the combination of aphotoanode and microbes. Until now it was the conventional thinking thatthe semiconductor electrode is more resistive compared to carbon ormetal electrodes, and that semiconductor materials are toxic to themicrobial communities when they are not stable in the microbial culturethat could be acidic, basic or redox active.

The current invention provides the combining of a photoanode withmicrobes within a single functional device by selecting thesemiconductor/microbe system carefully. FIG. 4A shows a schematicdrawing of the MPS configuration and the working mechanism of thebio-photoanode. In the dark, the semiconductor material simply functionsas a conductive electrode. The bio-electrons generated from bacteriaand/or electron shutters will transfer to the conduction band (CB) ofthe semiconductor, however, this electron transfer process is expectedto be slow due to the small potential difference between the redoxpotential of microbes and CB of the photoanode. In contrast, theinterfacial charge transfer can be substantially improved under lightillumination, which creates a lot of photoinduced holes in the low lyingsemiconductor valence band (VB). The recombination of bio-electrons withthese photoinduced holes is highly favorable due to the large potentialdifference (FIG. 4A, right). This process is facilitates thephotoinduced electron-hole separation and improves the yield of longlived photoinduced electrons for reduction reaction on the counterelectrode. Therefore, the photoanode functions as an electron sink toenhance the collection of bio-electrons.

Hematite (α-Fe₂O₃) has a favorable bandgap (2.1 eV) for solarabsorption, and it is photochemically stable in neutral/basic pH andneutral/oxidative conditions. Additionally, the material and productioncost of hematite (iron rust) is low, and it is biocompatible with mostcommon bacterial strains. Because poor electrical conductivity ofhematite affects the transport and collection of bio-electrons, hematitenanowires are implemented and chemically modified to increase theirelectrical conductivity, according to one embodiment of the invention.In one example, Shewanella MR-1 is used because it is a modelelectrogenic bacteria for bioelectricity production, and it is afacultative strain that does not require strict anaerobic growthenvironment. FIG. 4B shows a comparison of the linear sweepvoltammograms collected from a hematite based bio-photoanode device inthe presence of live bacteria, dead bacteria, and in the absence ofbacteria. Significantly, in the presence of live bacteria, the devicecurrent is substantially higher than the other two samples, in theentire potential window. The enhanced current is believed to be due tothe extra bio-electrons provided by the live bacteria. More importantly,these results show that a semiconductor can be used as anode forbacteria colonization, and the electron transfer between bacteria andphotoanode is possible. Moreover, the photoanode under lightillumination improves the collection of electrons from bacteria andelectron shutters, compared to the conventional carbon electrodes. Italso suggests that a smaller size of photoanode can potentially achievethe same charge collection efficiency as a large carbon electrode.

According to the current invention, the driving force for the chargetransfer under light illumination is related to the potential differencebetween the semiconductor VB and the bacteria oxidation potential.Bio-electrons generated by live bacteria are injected into the hematiteto recombine with the photoinduced holes, which block the electron-holerecombination path in hematite, and thus the excited state lifetime inthe presence of live bacteria is relatively longer lived compared tothat in the presence of dead bacteria and the absence of bacteria.Furthermore, electron-transfer rate constant (k_(et)=1/τ₂−1/τ₁) at theinterface of bacteria and hematite photoanode can be calculated based ontheir lifetimes.

From the hematite photoanode, other semiconductor materials forphotoanode are possible for achieving a goal of microbialelectrohydrogenesis at zero bias. To achieve spontaneous microbialelectrohydrogenesis, the CB of the semiconductor should be more negativethan the proton reduction potential (−0.41 V vs. normal hydrogenelectrode NHE at pH 7), while its VB should be more positive than thebacteria oxidation potential (see FIG. 5). It is noteworthy that directwater splitting is often limited by the very positive water oxidationpotentials as well as the high over-potential for oxygen evolvingreaction. Significantly, this harsh criterion can be relaxed in thepresence of bacteria because they have a much more negative oxidationpotential than water oxidation. Bacteria serve as a “bio-catalyst” foroxidizing organic matters in wastewater to produce electrons, which canfurther transfer to the VB of photoanode and recombined withphotoinduced holes. The current invention includes semiconductors withCB located at highly negative potential (compared to proton reductionpotential) that can achieve self-biased microbial electrohydrogenesis.

The current invention demonstrates for the first time a bio-photoanodefor spontaneous microbial electrohydrogenesis, which revolutionizes thedesign of MPS. By replacing the conventional carbon electrode with asemiconductor photoelectrode, the charge transfer and collectionefficiency at the interface between bacteria and electrode aresubstantially enhanced under light illumination. Furthermore, thebio-photoanode also provides a versatile platform for probing theelectron transfer rate of bio-electrons using time-resolved laserspectroscopy, which provides important insights into how the bacteriainteract with inorganic semiconductor materials, and its influence onthe charge collection efficiency. Finally, coupling the 3D electrodewith the bio-photoanode by growing semiconductor photoanode materials on3D carbon aerogel scaffold, the electrode offers the dual advantage oflarge surface area and light absorption capability.

The current invention directly interfaces the bio-photoanode with asemiconductor photocathode to form a “Z-scheme” MPS (see FIG.6). To theinventor's knowledge, the “Z-scheme” MPS has not been demonstrated yet.This embodiment has two benefits. First, the integration of twosemiconductor materials with optimal band-gap combination largelyimprove the solar light absorption. The addition of the photocathodealso offers extra photovoltage to the MPS. Second, the device cost isgreatly reduced by replacing the platinum counter electrode or platinumnanoparticle decorated carbon cloth electrode with low-costsemiconductor materials.

A dual-chamber MPS with a photocathode and a bio-photoanode separated bya proton exchange membrane (PEM) is provided, as shown in FIG. 6A. Aphotocathode is immersed in an anoxic buffered solution, and abio-photoanode preinoculated with electrogenic bacterial strains, forexample from wastewater, is used to generate electrons from organicwastes. The photoelectrodes are connected through an external circuit.In this exemplary device configuration, the expensive platinum electrodeor platinum coated carbon electrode is not required. Under lightillumination, photoexcited electron-hole pairs will be generated in bothanode and cathode. As shown in FIG. 6B, the bio-electrons recombine withthe photoinduced holes in the photoanode VB, while the photoinducedelectrons from photoanode diffuse to and recombine with the photoinducedholes in the photocathode VB. Finally the photoinduced electrons in thephotocathode CB reduce the protons to hydrogen. To balance the charge inthe chambers, the protons generated in the bio-oxidation reactionsdiffuse from the anode chamber through the PEM to the cathode chamber.On the whole, two photons generate two electron-hole pairs. Oneelectron-hole pair recombines, while the other pair is used foroxidation and reduction reactions. The total photovoltage of the MPS isthe combination of the photovoltage of anode and cathode.

For a spontaneous microbial electrohydrogensis process, in comparison tosingle band-gap MPS, the biggest advantage of Z-scheme MPS is theenhanced solar light absorption and photovoltage through the coupling oftwo distinct semiconductor photoelectrodes. Therefore the selection ofphotoelectrodes is critical to the overall performance of the MPS. Toachieve spontaneous microbial electrohydrogensis, the photocathode CBhas to be more negative than the proton reduction potential (−0.41 V vs.NHE at pH 7), and the photoanode VB should be more positive than thebacteria oxidation potential (−0.3-0V vs. NHE). The larger potentialdifference between these states will provide the larger driving forcefor the reactions. Furthermore, it is more favorable to have thesemiconductor electrodes with an optimal bandgap combination formaximizing the solar energy absorption. FIG. 7 shows the band-gap andrelative energies (vs. NHE) of several commonly used photoelectrodes. Anexemplary Z-scheme MPS using Fe₂O₃ (2.1 eV) nanowire film as photoanodeand Si (1.1 eV) nanowire array as photocathode is provided. Thephotoelectrodes are further decorated with low-cost Ni or NiFe basedcatalysts to reduce the over-potential for water oxidation and protonreduction. It is known that NiFe layered double hydroxide, exhibits highactivity toward both the oxygen and hydrogen evolution reactions. Thecombination of Fe₂O₃ and Si enables the MPS system to harvest most ofthe sunlight. And recent reports on the Fe₂O₃ photoanode and Siphotocathode for solar water splitting also strongly suggest that theyare very stable in aqueous solution and have outstanding photoactivity.

The invention demonstrates for the first time a Z-scheme MPS. Incomparison to previously developed solar-assisted microbial devices, theZ-scheme MPS significantly improve the utilization of solar energy.Moreover, the large photovoltage provided by the photoelectrodesfacilitate the charge transfer, and achieve spontaneous microbialelectrohydrogensis.

To achieve a sustainable and practically feasible MPS, it is critical tobuild the system based on low-cost earth abundant materials and tominimize the device fabrication and operation cost. In the existing MECdevices, the most costly components are platinum electrode or platinumdecorated carbon cloth electrode and cation/proton exchange membrane.According to the Z-scheme MPS embodiment, the platinum electrode isreplaced by a relatively low-cost semiconductor electrode, whichsubstantially reduces the device cost. In one embodiment the MPS isprepared in the form of powder, which can be dispersed into wastewaterto spontaneously generate hydrogen gas. In this embodiment, a“free-standing” MPS is provided that eliminates the cation/protonexchange membrane and the need of external circuit.

According to one embodiment, a free-standing MPS based on the Z-schemedevice configuration using p-Si nanowires and hematite nanoparticles asphotocathode and photoanode, respectively is provided. Both thephotocathode and photoanode are co-anchored on reduced-graphene oxide(rGO) sheets (see FIG. 8). Graphene oxide (GO) sheets are prepared fromlow-cost graphite powder using a modified Hummers and Offeman method.They are reduced by a number of methods to form highly conductive rGOsheets, which has relatively high electron mobility at room temperature.The rGO sheet functions as a substrate to host the semiconductorphotoelectrodes (photocatalyst), as well as an efficient electronshuttle to transfer bio-electrons generated in bacteria to photocathode.In comparison to the conventional MEC devices, the absence of a membraneresults in decreased internal resistance, and the reactions are longerlimited by the rate of ion diffusion. The assembly of rGO sheets withsemiconductor nanomaterials or bacteria (e.g., Shewanella) has beenseparately demonstrated. The current embodiment not only saves asubstantial amount of the material and device fabrication cost, but alsosimplifies the device operation and maintenance.

Since free-standing MPS is a fundamentally new device concept, weanticipate that there are several possible technical challenges. Herethe current embodiment addresses the following issues:

1) Possible contact between cathode and anode. The direct contact ofphotoanode and photocathode form a type II heterojunction, which causesthe failure of the mechanism and a significant energy loss throughelectron-hole recombination. this issue is addressed by using a two-stepgrowth approach. GO sheets are prepared by graphite powder using amodified method of Hummers and Offeman. p-Si nanowires are dispersedinto the GO solution. Under light illumination, the photoexcitedelectrons generated in Si nanowires reduce the surrounding GO to formrGO sheets, and the wire is eventually covered with rGO sheets. Thenn-Fe₂O₃ nanoparticles are deposited onto the as-synthesized Si/graphenecomposite by an appropriate coating method. In this case, anode andcathode are separated by the rGO sheet, and their inter-distance can beminimized.

2) Aggregation of graphene sheets. Photochemical stability is anotherimportant factor of the free-standing MPS. It is known that the graphenesheets are easily aggregate together due to its strong π-π interaction.The aggregation causes a significant drop of effective surface area, andthus, the efficiency of collecting bio-electrons and photocatalytichydrogen generation. Therefore, by immobilizing the graphene sheets ontoa transparent plastic substrate a thin graphene film is formed. Byputting the substrate in Si/rGO solution and allowing the solution toevaporate slowly, the rGO sheets adheres to the substrate to form thefilm. This aspect prevents the aggregation of graphene sheets as well aseliminates the tedious separation and recovery processes for the MPS.

3) Competing reactions in the wastewater. The composition of municipalwastewater is mainly water (>99.9%) and a complex mixture of differenttype of organic compounds and some inert inorganic solids. Since thereduction potentials of most organic compounds are higher than theproton reduction potential, they are not less favorable than thehydrogen evolution reaction (HER). The oxygen reduction reaction is themajor competing reaction to HER. Without an external supply of oxygen,the dissolved oxygen in the wastewater will be removed rapidly.

The present invention has now been described in accordance with severalexemplary embodiments, which are intended to be illustrative in allaspects, rather than restrictive. Thus, the present invention is capableof many variations in detailed implementation, which may be derived fromthe description contained herein by a person of ordinary skill in theart. For example, the invention can potentially be used for generationof liquid fuels and chemicals.

All such variations are considered to be within the scope and spirit ofthe present invention as defined by the following claims and their legalequivalents.

What is claimed:
 1. A wastewater to chemical fuel conversion device,comprising a housing, wherein said housing comprises a first chamber anda second chamber, wherein said first chamber comprises a bio-photoanode,wherein said second chamber comprises a photocathode, wherein a backsideof said bio-photoanode abuts a first side of a planatized fluorine dopedtin oxide (FTO) glass, wherein a backside of said photocathode abuts asecond side of said FTO glass, wherein a proton exchange membraneseparates said first chamber from said second chamber, wherein saidfirst chamber comprises a wastewater input and a reclaimed water output,wherein said second chamber comprises a solar light input and a H₂ gasoutput, wherein said solar light input is disposed for solar lightillumination of said first chamber and said second chamber.
 2. Thewastewater to chemical fuel conversion device of claim 1, wherein saidbio-photoanode comprises hematite (α-Fe₂O₃) nanowires.
 3. The wastewaterto chemical fuel conversion device of claim 1, wherein saidbio-photoanode comprises electrogenic bacterial strains.
 4. Thewastewater to chemical fuel conversion device of claim 1, wherein saidbio-photoanode comprises a semiconductor material selected from thegroup consisting of TiO₂, Fe₂O₃, WO₃, ZnO, and BiVO₄.
 5. The wastewaterto chemical fuel conversion device of claim 1, wherein said photocathodecomprises a semiconductor material selected from the group consisting ofInGaN, GaN, InP, GaP, Si, Cu₂O, and CuBi₂O₄.
 6. The wastewater tochemical fuel conversion device of claim 1, wherein said photocathode iscompatible with an anoxic buffered solution.